Michael Flynn, middle, is a central figure in the FBI’s investigation into whether the Kremlin worked with the Trump campaign to sway the US election.
Photograph: Carlos Barria/Reuters

A close associate of Donald Trump’s former national security adviser Michael Flynn arranged a covert investigation into Hillary Clinton’s use of a private email server when she was secretary of state, and through intermediaries turned to a person with knowledge of the “dark web” for help.

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Flynn, a retired three-star general who led chants of “lock her up” at last year’s Republican national convention, is a central figure in the FBI’s investigation into whether the Kremlin worked with the Trump campaign to sway the US election.

Flynn is personally and ideologically linked to Barbara Ledeen, a longtime conservative activist who works for the Republican senator Chuck Grassley on the Senate judiciary committee – which is now investigating alleged links between the Trump campaign and Russia.

Ledeen’s husband, Michael Ledeen, is also a confidant of Flynn, and co-authored a book with him last year.

Flynn was forced to resign in February after just 24 days on the job as Trump’s chief intelligence official in the White House, when it emerged that he had lied to Vice-President Mike Pence about conversations he had with the then Russian ambassador to the US, Sergey Kislyak.

According to interview notes released by the FBI last year, Ledeen decided in 2015 to launch her own investigation into Clinton’s use of the server. At the time, she was a staffer on the Senate judiciary committee.

According to the FBI files, Ledeen wanted to determine whether the emails had been hacked by a “foreign power”, because the incident angered her as a citizen and because she wanted to know whether such a hack would put her children, who served in the military, in danger.

Clinton’s use of a private server was steeped in controversy throughout her unsuccessful presidential bid, but Ledeen’s concerns proved to be unfounded. A federal investigation found no evidence that the emails on Clinton’s private server were ever compromised.

Ledeen’s name was redacted on the FBI documents describing the investigation, which were released last year in response to a Freedom of Information Act request. But a person who reviewed the unredacted documents confirmed to the Guardian that Barbara Ledeen was the subject. Her involvement was also confirmed by the Senate judiciary committee in response to the Guardian’s questions.

According to the FBI notes, Ledeen wanted to pursue her own investigation in 2015 into whether or not Clinton’s emails had been compromised but could not finance the work.

She sought out the help of an unnamed defense contractor and also turned to Newt Gingrich, the former Republican speaker of the House, for help. According to the FBI notes, Gingrich “wanted to speak to others about the project” and asked Judicial Watch, the conservative activist group, for financial assistance.

Judicial Watch allegedly turned to another, unnamed, contractor who was familiar with the “deep web and dark web”, according to the FBI files. The parties were concerned about what they would do if they came across any emails that contained classified information. According to the FBI investigation, the project was later halted.

Smith was convinced that Clinton’s private server had been hacked by a foreign power, probably Russians, Tait said.

Smith, who died at the age of 81 10 days after giving his own account to the Wall Street Journal, told the newspaper he had operated independently of the Trump campaign.

He allegedly told Tait that he had been approached by a person on the “dark web” who claimed to have a copy of emails from Clinton’s server and wanted help validating their authenticity.

According to Tait’s account, Smith claimed to be working with Flynn, who at the time was serving as a foreign policy adviser to Donald Trump’s presidential campaign.

Ledeen’s involvement is also important because she works on the Senate judiciary committee, which is conducting an investigation into the Trump campaign. Her family’s relationship with Flynn raises questions about whether Ledeen could be wielding influence over the investigation.

Grassley’s spokesman said that Ledeen’s 2015 inquiry had not been authorised by the judiciary committee and that the committee had only learned of it after it had been completed.

“She was instructed not to do any further follow-up once the committee learned of her involvement,” the spokesman said.

Congressional investigators do not have the power of the FBI and federal prosecutors to bring criminal indictments, but they can compel witnesses to testify publicly and under oath, and can potentially play an important role in setting the groundwork for impeachment proceedings against the president.

Grassley has several important decisions to weigh in how his investigation will proceed, including whether to call the president’s son, Donald Trump Jr, to testify publicly about a 2016 meeting he attended with Russians.

A Grassley spokesman told the Guardian that Barbara Ledeen was a part-time staffer on the judiciary committee judicial nominations unit. He said Ledeen was “in no way” connected to the investigations team and “would not have access to any of its materials”.

“Senator Grassley has no relationship with Barbara’s husband and wouldn’t recognise him if he saw him,” the spokesman added.

Ledeen and her husband have been influential – and controversial – players in conservative circles in Washington for decades.

Michael Ledeen, Barbara’s husband, is a historian and former Reagan administration official who helped to develop the secret programme to sell US arms to Iran in the late 1980s, in what is known as the Iran-Contra affair.